1. Field of the Invention
The present invention relates to a vertical amplifier system for a multitrace oscilloscope and method for calibrating the same that can equalize an input-output characteristic of each variable gain vertical amplifier of each channel.
2. Description of the Related Art
Variable gain amplifiers provided as vertical amplifiers in respective input circuits of a multitrace oscilloscope are adjusted so as to equalize their input-output characteristics. In other words, they are adjusted so that an identical input signal applied to any one of the channels is displayed in just the same form. This adjustment is generally carried out before delivery or at the time of maintenance.
FIG. 1 is a block diagram showing an arrangement of a conventional calibration circuit for a vertical amplifier system. In FIG. 1, reference numeral 1 designates a signal input terminal of a channel 1 (hereinafter referred to as CH1); 2, a signal input terminal of a channel 2 (hereinafter referred to as CH2); from 3 to 6, variable attenuators; 7 and 8, buffer amplifiers; 9 and 10, variable gain amplifiers each of which outputs an output current in proportion to an input voltage; 30, a multiplexer which selects a signal to be displayed on a cathode ray tube (hereinafter referred to as CRT) 11 in response to a selection signal SSO; 12, an amplifier to drive the CRT 11; 13, a gain adjustor for the amplifier 12; 14, an output offset adjustor for the amplifier 12; 15, a resistor to detect an input current fed to the amplifier 12; and 16, an analog-to-digital converter which converts the detected current to a digital signal. The output digital signal of the analog-to-digital converter 16 is transferred to a CPU 18 through a bus gate 17. Reference numeral 19 denotes a memory to store data; 20, an input signal selector which selects a signal supplied to each variable gain amplifier 9 or 10 in response to a selection signal SSI; 21, a reference voltage generator which generates a first reference voltage Vref to calibrate the variable gain amplifiers 9 and 10; 22-25, latches; and 26-29, digital-to-analog converters. The CPU 18 feeds gain control signals GC1 and GC2, or position (offset) control signals PC1 and PC2 to the variable gain amplifiers 9 and 10 via the latches 22-25 and the digital-to-analog converters 26-29. Here, the gain control signals GC1 and GC2 are for controlling the gains of the variable gain amplifiers 9 and 10, and the position control signals PC1 and PC2 are for adjusting the offsets of the output currents of the variable gain amplifiers 9 and 10.
The conventional circuit arranged as in FIG. 1 operates differently in a normal mode where input signals are displayed and in a calibration mode where the gains and offsets of the variable gain amplifiers are adjusted. In the normal mode, the CPU 18 controls the input signal selector 20 to select its terminals d and d' by using the selection signal SSI. Thus, signals inputted to the terminals 1 and 2 to be measured, are fed to the variable gain amplifiers 9 and 10. In addition, the CPU 18 controls the multiplexer 30 so that the multiplexer 30 alternately selects its terminals a and a', and b and b' in response to the selection signal SSO, thus, displaying the signals to be measured on the CRT 11.
The operation of the calibration mode of the vertical amplifier system comprising the variable gain amplifiers 9 and 10 is as follows:
When the variable gain amplifiers 9 and 10 are calibrated, the CPU 18 controls the input signal selector 20 to select its terminals e and e' by using the selection signal SSI. The variable attenuators 5 and 6 are set at a certain fixed attenuation ratio. In this condition, the first reference voltage Vref is supplied to the variable gain amplifiers 9 and 10.
In the calibration mode of the variable gain amplifier 9, the CPU 18 controls the multiplexer 30 to select its terminals a and a' at first by using the selection signal SSO in order to detect the output current I1 of the variable gain amplifier 9. Then, as shown in FIG. 2, the inclination of the input-output characteristic line is adjusted by using the gain control signal GC1 so that the output current I1 becomes a predetermined current I0.
Next, the CPU 18 controls the input signal selector 20 to select its terminals f and f' by using the selection signal SSI so that a ground signal (0 V) is supplied to the variable gain amplifier 9 as a second reference voltage for an offset adjustment. Subsequently, as shown in FIG. 2, a position (offset) I.sub.f of the input-output characteristic line is adjusted by the position control signal PC1 so that the value of output current I1 becomes zero.
As clearly shown in FIG. 2, when the gain of the variable gain amplifier 9 is changed so that the output current of the variable gain amplifier 9 becomes I0 when the input voltage is Vref, the output current (offset) I.sub.f corresponding to the second reference voltage (0 V) is also changed. Reversely, when the position (offset) I.sub.f is changed when the second reference voltage is inputted, the output current corresponding to the input voltage Vref is also changed. Accordingly, the CPU 18 must repeat alternately adjusting the gain and the position (offset) until the input-output characteristic line accords with the characteristic line indicated by a broken line in FIG. 2. At this stage, the calibration of the variable gain amplifier 9 is completed.
Subsequently, the CPU 18 controls the multiplexer 30 to select its terminals b and b' by using the selection signal SSO, thus calibrating the variable gain amplifier 10 by the same method as used for calibrating the variable gain amplifier 9.
Thus, both variable gain amplifiers 9 and 10 have the same input-output characteristic as shown by the broken line of FIG. 2 after calibration.
Here, an operator manipulates a switch connected to the CPU 18 so that the multiplexer 30 selects one of two outputs of the variable gain amplifiers 9 and 10, and the selected output is fed to the amplifier 12 to drive the CRT 11. Subsequently, the operator manipulates a switch connected to the CPU 18 so that the input selector 20 selects the ground signal (0 V) (the second reference voltage), and adjusts the output offset with adjustor 14 while observing a screen so that the signal is displayed at a first intended position of the screen of the CRT 11. Next, the operator manipulates the switch connected the CPU 18 so that the input selector selects the first reference voltage Vref, and adjusts the gain adjustor 13 so that the signal is displayed at a second intended position on the screen. Finally, the operator repeats the foregoing adjustment until the ground signal (the second reference voltage) and the first reference voltage Vref are correctly displayed at the first and second intended positions, respectively.
Thus, a gain and an output offset of the variable gain amplifiers in a whole vertical amplifier system are finally decided so that the input signal is displayed correctly with matching the characteristics of an individual CRT by manipulating the gain adjustor 13 and the output offset adjuster 14.
Incidentally, accuracy of calibrating the variable gain amplifiers 9 and 10 is determined by the resolution of the analog-to-digital converter 16. If a tolerance determined by the resolution of the analog-to-digital converter is .+-..epsilon., the input-output characteristics of both variable gain amplifiers 9 and 10 may include errors within the range shown in FIG. 3 in the foregoing conventional circuit. That is, the output offsets and the gains of the two variable gain amplifiers may include difference up to 2.epsilon. and 4.epsilon./Vref, respectively.
Furthermore, in the foregoing conventional circuit, there is a problem that both input-output characteristics of the variable gain amplifiers 9 and 10 do not match each other even after the adjustment, when the output voltage of the reference voltage generator 21 is changed during the calibration interval of the variable gain amplifiers 9 and 10. In other words, waveforms on the screen of CRT 11 may be different even when an identical signal is inputted to each channel after adjustment.